U.S. patent number 5,233,985 [Application Number 07/730,160] was granted by the patent office on 1993-08-10 for cardiac pacemaker with operational amplifier output circuit.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Terrence R. Hudrlik.
United States Patent |
5,233,985 |
Hudrlik |
August 10, 1993 |
**Please see images for:
( Certificate of Correction ) ** |
Cardiac pacemaker with operational amplifier output circuit
Abstract
A medical electrical stimulator employing an operational
amplifier output circuit for producing an electrical stimulating
pulse for application to body tissue and for sensing electrical
activity in the body tissue. A first input to the operational
amplifier is coupled through a virtual load to a probe electrode in
close proximity to the body tissue. The second input is coupled to
a second electrode which may be remote from the tissue to be
stimulated. A defined voltage signal may be provided to the second
input to the amplifier, and the amplifier correspondingly delivers
current through the virtual load to the probe electrode as the
amplifier maintains equal voltage levels at its two inputs. The
current delivered to the probe electrode functions to stimulate the
body tissue. By varying the defined voltage signals provided to the
second input of the amplifier, arbitrary stimulation pulse
waveforms may be generated. After termination of the defined
voltage signal, the amplifier functions to restore the
electrode/tissue system to its previous electrical equilibrium
condition and to sense induced or spontaneous electrical activity
in the tissue. The circuit may be employed in cardiac pacemakers,
with the probe electrode located on or in the heart, or in other
electrical medical stimulators.
Inventors: |
Hudrlik; Terrence R. (Fridley,
MN) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
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Family
ID: |
27415996 |
Appl.
No.: |
07/730,160 |
Filed: |
July 15, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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566636 |
Oct 10, 1990 |
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827858 |
Jan 30, 1992 |
5156149 |
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626061 |
Dec 12, 1990 |
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Current U.S.
Class: |
607/27 |
Current CPC
Class: |
A61N
1/37 (20130101); A61N 1/371 (20130101); A61N
1/3706 (20130101); A61N 1/3704 (20130101) |
Current International
Class: |
A61N
1/362 (20060101); A61N 1/37 (20060101); A61N
001/362 () |
Field of
Search: |
;128/419 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Duthler; Reed A. Patton; Harold
R.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
Attention is drawn to the commonly assigned copending U.S. Patent
Application Ser. No. 07/566,636, for a "Field Density Clamp for
sensing Cardiac Depolarizations", filed Oct. 8, 1990 in the name of
Terrence R. Hudrlik now abandoned and replaced by U.S. patent
application Ser. No. 07/827,858, filed Jan. 30, 1992, issued as
U.S. Pat. No. 5,156,149 and U.S. patent application Ser. No.
07/626,061, "Electronic Capture Detection for a Pacer", filed Dec.
12, 1990 in the name of Terrence R. Hudrlik, both of which are
incorporated herein in by reference in their entireties. The
present application is a continuation in part of both of these
cited applications.
Claims
In conjunction with above specification, I claim:
1. A stimulation pulse generator apparatus for a medical electrical
stimulator, comprising:
an operational amplifier having an amplifier output and having
first and second amplifier inputs; first electrode electrically
coupled to said first input of said amplifier;
a virtual load electrically coupled between said first electrode
and said first input of said amplifier;
a second electrode coupled to said second input of said amplifier;
and
voltage source means for providing defined pulsatile voltage
waveforms to said second input of said amplifier to cause
corresponding delivery of voltage pulses to said first
electrode.
2. A medical electrical stimulator apparatus, comprising:
a timing circuit for triggering the generation of stimulation
pulses; and
a pulse generator triggered by said timing circuit to generate
stimulation pulses, said pulse generator comprising an operational
amplifier having an amplifier output and having first and second
amplifier inputs;
a first electrode electrically coupled to said first input of said
amplifier;
a virtual load electrically coupled between said first electrode
and said first input of said amplifier; second input of said
amplifier; and
voltage source means for providing defined pulsatile voltage
waveforms to said second input of said amplifier to cause
corresponding delivery of voltage pulses to said first
electrode.
3. A cardiac pacer apparatus, comprising:
a resettable timing circuit for triggering the generation of
stimulation pulses;
a pulse generator triggered by said timing circuit to generate
stimulation pulses, said pulse generator comprising an operational
amplifier having an amplifier output for providing electrical
signals and having first and second amplifier inputs;
a first electrode electrically coupled to said first input of said
amplifier;
a virtual load electrically coupled between said first electrode
and said first input of said amplifier;
a second electrode electrically coupled to said second input of
said amplifier; and
voltage means for providing defined pulsatile voltage waveforms to
said second input of said amplifier to cause corresponding delivery
of voltage pulses to said first electrode; and
means for detecting a heart depolarization and for resetting said
timing circuit in response to detection of a said depolarization,
said detecting means responsive to electrical signals provided by
said output of said operational amplifier.
4. An apparatus according to claim 3 wherein said detecting means
comprises means for defining a capture detection time interval
following provision of a said pulsatile voltage waveform to said
second input of said operational amplifier and means responsive to
detection of a said depolarization occurring during said capture
detection time interval for indicating that the said corresponding
voltage pulse delivered to said probe electrode was successful in
stimulating a heart depolarization.
5. The apparatus of claim 1 or claim 2 or claim 3 or claim 4
wherein said first electrode is adapted for implantation on or in
the heart and wherein said first electrode has a surface area of 5
square millimeters or less, and wherein said virtual load has a
resistance of 1,000 ohms or less.
6. Apparatus for detecting and stimulating depolarization of
cardiac tissue, comprising:
an active circuit having first and second inputs and an output and
having means for maintaining said first and second inputs at the
same voltage;
a first electrode;
a virtual load coupled between said first electrode and said first
input;
a second electrode coupled to said second input; and
voltage source means for providing pulsatile voltage signals having
defined waveforms to said second input;
wherein said maintaining means comprises means for maintaining said
first and second inputs at the same voltage by delivering
electrical energy through said virtual load and said first
electrode and wherein said output of said active circuit provides a
signal indicative of the electrical energy delivered through said
virtual load.
7. Apparatus for stimulating tissue, comprising:
an active circuit having first and second inputs and an output and
having means for maintaining said first and second inputs at the
same voltage;
a first electrode;
a virtual load coupled between said first electrode and said first
input;
a second electrode coupled to said second input; and
voltage source means for providing pulsatile voltage signals having
defined waveforms to said second input;
wherein said maintaining means comprises means for maintaining said
first and second inputs at the same voltage by delivering
electrical energy through said virtual load and said first
electrode, whereby a voltage pulse is delivered to said first
electrode in response to the provision of a pulsatile voltage
signal by said voltage source means.
8. The apparatus of claim 7 wherein said active circuit comprises
monitoring circuitry responsive to the electrical energy delivered
through said virtual load and providing said output signal in
response to a said delivered energy meeting predetermined
criteria.
9. The apparatus of claim 8 wherein said monitoring circuitry
comprises means for defining a capture detection time interval
following provision of a said pulsatile voltage signal to said
second input of said active circuit and means responsive to said
output signal occurring during said capture detection time interval
for indicating that the said voltage pulse delivered to said probe
electrode was successful in stimulating said tissue.
10. The apparatus of claim 6 or claim 7 or claim 8 or claim 9
wherein said first electrode is adapted for implantation on or in
the heart.
11. The apparatus of claim 10 wherein said first electrode has a
surface area of 5 square millimeters or less, and wherein said
virtual load has a resistance of 1,000 ohms or less.
12. The apparatus of claim 6 or claim 7 or claim 8 or claim 9
wherein said active circuit comprises an operational amplifier.
Description
BACKGROUND OF THE INVENTION
This invention relates to electrical tissue stimulators and more
particularly, to tissue stimulator output circuits and sense
amplifier circuits.
Electrical body tissue stimulators, such as nerve or muscle
stimulators, cardiac pacemakers and the like are well known in the
prior art. Electrical energy has been applied to body tissue using
batteries, condensers, electrostatic charge generators and
alternating current generators either for scientific curiosity or
for treatment of various injuries and illnesses. Condenser
discharge impulses of electrical energy into body tissue have been
used therapeutically since the invention of the Leyden jar and
their use continues today in implantable body stimulators,
particularly cardiac pacemakers.
Early implantable cardiac pacemakers, such as those shown in U.S.
Pat. No. 3,057,356 and subsequent pacemakers up to the present date
comprise small, completely implantable, transistorized and battery
operated pulse generators connected to flexible leads bearing
electrodes directly in contact with cardiac tissue. Demand cardiac
pacemakers have traditionally employed a timing circuit, a
stimulating circuit and a separate sensing circuit, all of which
draw current from the power source. The stimulating circuits of
such pulse generators have traditionally comprised constant current
or constant voltage output circuits employing output capacitors
which are charged to battery potential through a relatively high
impedance and discharged through electrodes in contact with
myocardial tissue to stimulate depolarization of the tissue. The
output capacitors are typically recharged during the intervals
between successive discharges.
The discharge of an output capacitor through the myocardial tissue
results in after-effects due to the disruption of the electrical
equilibrium condition at the tissue-electrode interface and the
polarization of the tissue's intrinsic dipole moments. Post
relaxation of these stimulation based after-effects, traditionally
characterized as "polarization", manifest themselves to traditional
pacemaker sense amplifiers coupled to the stimulation electrode as
voltage signals which persist for a period of time following
delivery of stimulation pulses. In traditional pacemakers, these
after-effects interfere with the pacemaker's ability to sense
depolarizations of the heart during, closely following or caused by
delivery of stimulation pulses.
Various attempts were made in the prior art to counteract the
"polarization" after-effects of the stimulation pulse and
simultaneously recharge the output capacitor by means of a fast
recharge pulse delivered through the stimulation electrodes
following the trailing edge of the output pulse, as exemplified by
U.S. Pat. Nos. 4,476,868, 4,406,286, 3,835,865 and 4,170,999.
However, simply passing sufficient current through the
electrode-tissue interface to recharge the output capacitor does
not necessarily return the electrodetissue system to its prior
electrical equilibrium condition. Alternatively, it has been
suggested to counteract the after-effects of delivery of a
stimulation pulse by simply tying the electrodes involved in
delivery of the pulse together following delivery of the pulse, as
disclosed in U.S. Pat. No. 4,498,478 issued to Bourgeois or by
means of a train of low energy pulses as disclosed in U.S. Pat. No.
4,811,738, issued to Economides, et al.
SUMMARY OF THE INVENTION
The method and apparatus of the present invention contemplates the
use of the field density clamp operational amplifier both to effect
body tissue stimulation and to sense electrical activity in the
body tissue. Therefore it is an object of the present invention to
provide a stimulating pulse output circuit which is capable of
stimulating body tissue, particularly myocardial tissue, sufficient
to effect a desired tissue response, such as depolarization, while
reducing post pulse disturbances of the electrode/tissue
equilibrium condition normally present between stimulation pulses
and/or tissue depolarizations.
It is a further object of the present invention to provide a
simplified stimulation pulse generator which eliminates the output
capacitor and avoids the complexity of circuit components which
have been provided to correct for or compensate for "polarization"
effects in the prior art. Elimination of the traditional output
capacitor provides an additional substantial benefit in that
arbitrary output waveforms may be defined an applied to the
exitable tissue simply by varying the defined controlling voltage
signal. Extended pulse waveforms, ramped voltage waveforms, and
constant voltage waveforms, for example, may all readily be
accomplished. Both suprathreshold and subthreshold pulses may be
generated and delivered.
The ready adjustability of the stimulation pulse waveform in
conjunction with the ability to sense tissue depolarizations
closely following delivery of the stimulation pulse also provide a
sensing and stimulating system which is beneficially used in a
pacemaker as 07/626,061, by Hudrlik. This application also
discloses a pacemaker which monitors the success of the stimulation
pulses in capturing the heart tissue and adjusts the stimulation
pulse energy accordingly.
It is still further an object of the present invention to eliminate
or reduce the necessity for providing blanking and refractory
intervals during and following the delivery of a stimulating output
pulse, particularly in the context of single and dual chamber
cardiac pacing systems.
It is still a further object of the present invention to employ the
amplifier as both the sense amplifier for sensing natural
depolarizations of the heart and as the output pulse generator of a
body tissue stimulator, such as a cardiac pacemaker.
These and other objects of the present invention are realized in
the output pulse stimulating circuit of the present invention which
comprises an operational amplifier having first and second input
terminals and an output terminal, a feedback resistor coupled
between the first input terminal and the output terminal of the
operational amplifier, a first, probe electrode adapted to be
placed in contact with tissue to be stimulated and coupled by a
first lead to the first (negative) input terminal of the
operational amplifier through a virtual load resistance, a second
electrode adapted to be coupled to body tissue and coupled to the
second (positive) input terminal of the operational amplifier, and
a source for a defined voltage signal which may be applied to the
second input to the amplifier. The defined voltage signal, when
applied to the second input of the amplifier, forces the first
input of the amplifier to the defined voltage. The first input to
the amplifier is foroed to the defined voltage by means of current
applied through the feedback and virtual load resistor. This
feedback process reproduces and maintains the defined voltage at
the first (negative) input of the operational amplifier and this
voltage, as applied to the probe electrode comprises the
stimulation pulse.
In the context of the present invention, the virtual load impedance
may be chosen to provide a low input impedance to the amplifier,
for example 100 ohms or less. The virtual load impedance and the
capacitive and resistive characteristics of the tissue-electrode
system, define the current provided to the probe electrode as a
function of the defined voltage signal.
The virtual load impedance may be adjusted to vary the sensing
characteristics of the amplifier as disclosed in the above cited
application Ser. No. 07/566,636. The stimulation pulse
characteristics may be varied by adjusting the defined voltage
signal provided to the second input of the amplifies and by
adjustment of the virtual load and feedback impedances. By
adjusting these parameters, a wide variety of sensing and
stimulation characteristics can easily be obtained, and the device
may be optimized for use with electrodes of varying types.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and still further objects, features and advantages of the
present invention will become apparent from the following detailed
description of a presently preferred embodiments, taken in
conjunction with the accompanying drawings, and, in which:
FIG. 1 is a schematic diagram depicting the interconnection of a
pacemaker pulse generator and pacing lead with the heart;
FIG. 2 is a schematic diagram illustrating a field density clamp
amplifier configured for use as a sense amplifier only.
FIG. 3 is a schematic diagram depicting a first embodiment of the
present invention in which the amplifier of the present invention
is used as both a sense amplifier and the output circuitry for a
cardiac pacemaker system;
FIG. 4 is a block diagram depicting an auto-threshold cardiac
pacemaker employing the circuitry illustrated in FIG. 3;
FIG. 5 is a block diagram depicting an alternative embodiment of a
cardiac pacemaker employing the circuitry illustrated in FIG. 3;
and
FIG. 6 is a set of real time ECG tracings taken in the laboratory
and associated timing diagrams illustrating the operation of the
pacemaker illustrated in FIG. 4 in conjunction with detection of
depolarizations induced by stimulus pulses.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following description, reference is made to illustrative
embodiments for carrying out the invention. It is understood that
other embodiments may be utilized without departing from the scope
of the invention. For example, the invention is disclosed in the
context of a VVI single chamber pacemaker system for treating
bradycardia. It should be appreciated that the technique for
myocardial depolarization stimulation and detection could also be
applied to other forms of pacemakers and heart stimulators
including dual chamber pacemakers (DDD, VDD, DVI, etc), rate
responsive pacemakers (single and dual chamber) and
antitachyarrhythmia devices. The detection of signals from and/or
the stimulation of other body tissue than myocardial tissue may
also be accomplished using the present invention. For example, the
concepts of the present invention may be employed in electrical
stimulation systems for stimulating other organ and skeletal muscle
systems and/or the nervous system of a patient.
FIG. 1 is a representation of an implanted pacemaker 14 in relation
to a lead system 12 and heart 10. Typically, the pacemaker 14 is
implanted beneath the patient's skin, outside the rib cage in the
pectoral region. A pacing lead 12 is passed pervenously through the
right atrium and into the right ventricle of the heart 10. The
pacing lead 12 is used for supplying pacing pulses to the heart and
conducting electrical signals resulting from depolarizations of the
heart tissue to the pacemaker 14.
There are two basic sensing configurations which may be employed
using pacing lead 12. A unipolar electrode configuration would
employ tip electrode 22, referenced to case electrode 24. Typically
the distance between the distal tip electrode 22 and the pacer case
electrode 24 is between 10 and 30 cm. A bipolar electrode
configuration would employ ring electrode 21 and tip electrode 22.
Typically, the tip and ring electrodes 22 and 21 are spaced apart
between 0.5 and 3.0 cm. In dual chamber pacemakers, electrodes for
unipolar and/or bipolar sensing are similarly situated on or in the
atrium or coronary sinus. FIG. 2 discloses an amplifier for use in
conjunction with the present invention. This form of amplifier is
also described in the above-cited co-pending patent applications by
Hudrlik, incorporated by reference in their entirety. The active
circuitry of the amplifier 38 attempts to maintain equal voltage
levels at its two inputs. Passage of a depolarization wavefront
changes the distribution of electrical charges and the electric
field in the vicinity of the electrode 22. This disturbance results
in the active circuitry of the amplifier delivering current through
feedback resistor 48 and virtual load resistor 44 to maintain equal
voltages at its inputs. This current, delivered to electrode 22,
serves both to reestablish the equilibrium condition in effect
preceding the passage of the depolarization wavefront and to signal
the occurrence of the depolarization wavefront.
As shown in the schematic diagram of FIG. 2, the sense amplifier
may be practiced with an operational amplifier 38 which has its
non-inverting input 40 connected to the can electrode 24. The
inverting input 42 is coupled to tip electrode 22 through a
variable resistor 44 which is used to set a virtual load resistance
for the system. This resistance is preferably between 10 and 1000
ohms, and is preferably less than 100 ohms for use in conjunction
with small surface area electrodes, typically about 5 square
millimeters or less in surface area.
The inventor has determined that when an amplifier according to the
present invention is coupled to a canine heart by means of a
polished platinum probe electrode, R-waves exhibited during normal
sinus rhythm place a peak demand for current through the virtual
load of about 0.5 microamps per square millimeter of electrode
surface. It is preferred that the peak current demand fall in the
vicinity of 2.5 microamps or less, which can be accomplished with a
platinum electrodes of about 2-5 square millimeters in surface
area. Electrodes fabricated of other metals will have differing
current requirements and therefore will have differing optimal size
ranges. Electrodes directly in contact with the myocardium will
typically require a greater peak current per square millimeter of
surface area and therefore will typically have somewhat smaller
optimal surface areas or will employ current shunts to redirect the
excess current.
A feedback path is provided for the amplifier 38 by a feedback
resistor 48 which defines a voltage signal B on line 39
proportional to the current through the virtual load 44 and
feedback resistor 48. A differential amplifier 54 may optionally be
provided to measure the magnitude of the potential difference
between electrodes 22 and 25, and thus the voltage across the
virtual load 44. The non-inverting input 50 of this differential
amplifier 54 is coupled to tip electrode 22 while the can electrode
24 is coupled to inverting input 52. The voltage output A of
differential amplifier 54 is proportional to the voltage across the
virtual load resistor 44.
The voltage measurement A and the current measurement B may be used
to compute the power delivered through the virtual load as a result
of the passage of a cardiac depolarization wavefront. Detection of
the passage of the depolarization wavefront based on measured power
delivered through the virtual load may be employed for sensing
purposes in the context of the present invention. However it is
also workable to use the current signal B, alone, to detect the
depolarization, and in the specific embodiments discussed below,
only this signal is employed.
The power computation is carried out by an analog multiplier 56
which computes the power level an provides a voltage output C
proportional to the computed power. Current signal B or power
signal C is communicated to comparator 58 via switch 57. Comparator
58 compares the selected input to a threshold voltage VREF defined
by voltage source 46. If the selected one of the current signal B
or the power signal C exceeds Vref, comparator 8 generates a
V-sense detect signal VSD on line 32.
FIG. 3 is a schematic diagram illustrating one embodiment of a
combined input/output stage employing the field density clamp
amplifier illustrated in FIG. 2. Operational amplifier 100 has its
negative input coupled to probe electrode 22, through virtual load
resistor 104. Load resistor 104 is shown as an adjustable resistor,
an adjustment of load resistor 104 allows for tuning of the sense
amplifier, as discussed in the above-cited Hudrlik application,
Ser. No. 07/566,636. By reducing the application impedance of
virtual load 104, the signal contribution of the heart tissue
remote from electrode 22 is diminished, and the relative
contribution of tissue in the immediate vicinity of electrode 204
is increased. For purposes of the present invention, a virtual load
impedance of 100 ohms or less is believed to be preferable, with
the virtual load impedance 104 as close to zero as is practicable.
Indifferent electrode 24, which may take the form of all or a
portion of the can of the pacemaker is coupled to the positive
input of operational amplifier 00 through an adjustable resistor
134. Feedback resistor 102 defines a voltage at the output of
amplifier 00 proportional to the current delivered through virtual
load resistor 104. Operation of amplifier 100 to sense cardiac
depolarizations corresponds to the operation of amplifier 38,
discussed above in FIG. 2, and discussed extensively in the
above-cited Hudrlik applications.
The output of amplifier 100 is coupled to the input of differential
amplifier 106, which operates as an adjustable gain stage of
conventional design, with gain being controlled by variable
resistor 108. Operational amplifier 112 controls the offset of
amplifier 106, which may be adjusted by means of a variable voltage
provided by variable resistor 114. The output of amplifier 106 is
provided to an amplifier output line 118, for use as an analog
signal, if desired. The output of amplifier 106 is also provided to
detection block 116, which detects the occurrence of a signal from
amplifier 106 that exceeds a predetermined sensing threshold value.
This threshold value may be a simple voltage level threshold or may
be the composite output from a convolution based threshold
detector.
Detection block 116 may correspond to circuitry used to establish
sensing thresholds in any prior art pacemaker, and is illustrated
functionally herein for that reason. In response to the output
signal from amplifier 106 exceeding a predetermined threshold,
positive or negative, a sense detect signal (SD) is generated on
line 32. In order to prevent sense detect signals from being
generated in response to delivery of the pacing pulse itself, the
detection block may be inhibited during the pacing pulse and for
the next few milliseconds thereafter by means of a signal on INH
line 38. If the amplifier is not being used to perform capture
detection as discussed below, the signal on INH line 38 may persist
for up to 100 milliseconds after the pacing pulse, corresponding to
the digital blanking intervals used on many prior art pacers.
Alternatively, if amplifier 26 is being used to perform capture
detection, the signal on INH line 38 may persist only long enough
to allow the amplifier 100 to restore the equilibrium condition at
the electrodes, e.g for about 5 ms.
Use of operational amplifier 100 to deliver a stimulation pulse is
accomplished by imposing a predefined voltage at the positive input
of amplifier 100, as discussed above. Operational amplifiers 124
and 130 in conjunction with associated resistors 120, 122, 126, 128
and 132 function to provide an adjustable, controlled current
through resistor 134 as a function of the voltage applied to the
negative input of amplifier 124 on line 47, marked "VIN". The
current through resistor 134 is defines a voltage signal provided
to the positive input of operational amplifier 100, triggering
current flow through feedback resistor 102 which drives the
inverting input of operational amplifier 102 to the same voltage as
applied to the non-inverting input. This virtual node voltage (the
voltage at the inverting input to amplifier 102) is applied across
virtual load resistor 104 and probe electrode 22 to stimulate the
heart.
Adjustment of the voltage signal provided to amplifier 100 may be
accomplished by means of adjustment of resistor 132, resistor 134,
or by variation of the signal provided via VIN line 47. Generally,
the output 5 circuitry illustrated in this figure responds to a
voltage on VIN line 47 by producing a current through virtual load
resistor 104 sufficient to maintain the inputs to amplifier 100 at
the same voltage.
In the context of cardiac stimulation, it is envisioned that square
waves of 2 milliseconds or less in duration will generally be
applied to the negative input of amplifier 124, to trigger voltage
pulses applied to electrode 22. However, ramped voltage waveforms,
sinusoidal voltage waveforms or arbitrary voltage waveforms may
also be provided to amplifier 124, with corresponding voltage
waveforms generated by amplifier 100. Circuitry for generating
square voltage pulses, ramped voltage pulses, sinusoidal voltage
pulses and/or other arbitrary voltage wave pulses may be employed
to define the stimulus current waveform. Such circuitry is believed
well known to the art, and is therefore not disclosed in detail
herein. For most pacing purposes, it is envisioned that simple
rectangular voltage pulses will be applied to VIN line 47, with
either pulse amplitude or pulse duration increased in order to
increase the energy level of the stimulation pulse applied to
electrode 22 by amplifier 100.
As discussed above, following delivery of the stimulation pulse to
electrode 22, amplifier I00 delivers current through load resistor
104 to counteract the "polarization" aftereffects associated with
delivery of the stimulation pulse (potential present between the
electrodes following the application of a stimulation pulse), and
to rapidly restore the electrode-tissue system to its previous
equilibrium condition. Following delivery of the stimulating pulse,
amplifier 100 may quickly (within 10 ms or less) be used for
sensing of the occurrence of a depolarization induced by the
stimulating pulse and for sensing natural depolarizations of the
heart tissue.
Because amplifier 100 is active during delivery of the stimulation
pulse, a voltage indicative of the stimulation current will appear
at its output. As such, the circuit illustrated provides a ready
means of measurement of characteristics associated with the
electrode-tissue interface and of the integrity of the pacing lead.
The signal indicative of the current delivered to electrode 22, as
amplified by amplifier 106 may be passed along to recording and
analysis circuitry, if provided, on line 11B. Similarly, because
the amplifier 100 is active continually from delivery of the pacing
pulse, the current through virtual load resistor 104 immediately
following the stimulation pulse to counteract "polarization" after
effects may also be passed through amplifier 106 to line 118, for
recording and analysis. The current delivered to electrode 22 in
the first few milliseconds following delivery of the stimulation
pulse may provide information with regard to the condition of the
tissue adjacent the electrode 22, or other useful information.
The sensed induced depolarization and natural depolarization
wavefronts, of course, also result in corresponding voltage signals
at the output of amplifier 100, which may be used for detection of
depolarizations using the detection circuitry 116 or may be
recorded and analyzed employing any of the waveform analysis
techniques known to the art, including measurement of the
amplitude, width, slew, etc., of the voltage signal associated with
the detected depolarization. This form of analysis is believed to
be particularly valuable in conjunction with the use of the present
invention in the context of an implantable tachyarrhythmia device,
where waveform analysis is likely to be of significance in
distinguishing naturally conducted and aberrantly conducted
depolarization wavefronts.
FIG. 4 depicts the major circuit elements contained within a
pacemaker employing the present invention and adapted to detect
whether the delivered stimulation pulses are successful in
capturing the heart. The sense amplifier 26 is coupled to sense
electrical heart signals between the tip electrode 22 and the can
electrode 24. The pacing pulse waveform generator 34 is preferably
connected to the sense amplifier 26, and serves to define the
stimulation pulse waveform as discussed above.
A second field density clamp sense amplifier 27 is also
illustrated, coupled to the ring electrode 21 (FIG. 1) and to the
pacer can 24. Amplifier 27 may correspond to the sense amplifier
illustrated in FIG. 2 and is used to perform the capture detection
function as disclosed in the above cited Hudrlik application for a
"Electronic Capture Detection for a Pacer".
In operation, the sense amplifier 26 detects the occurrence of a
cardiac depolarization, and in response generates a sense detect
signal (SD) on line 32. The occurrence of an SD signal resets the
escape interval timer 30 and thus resynchronizes the pacer to the
underlying rhythm of the patient's heart. If no ventricular
depolarizations are sensed within the escape interval, timer 30
generates a ventricular pace signal on line 29 at the expiration of
the escape interval. The ventricular pace signal (VP) is provided
to the pacing pulse waveform generator circuit 34 via line 36 and
triggers generation of a predefined voltage signal as discussed
above which controls the current provided to the probe electrode
22. Typically, the escape interval timer 30 is remotely programmed
by telemetry to adjust the duration of the ventricular escape
interval, which corresponds to the desired maximum time interval
between heartbeats.
The VP signal on line 36 generated by the escape interval timer 30
is also communicated to electronic capture detect timer 33 via line
49. The VP signal resets timer 33, which thereafter defines the
capture detect time window. During the capture detect window (T2),
timer 33 provides a signal on line 43 which enables gate 41. The
occurrence of an SD signal from amplifier 26 or amplifier 27 during
the capture detect window results in a capture detect signal (ECD)
from gate 41 on line 37.
In the case of a typical modern pacemaker, the duration of the
pacing pulse may be about 1 ms or less, the amplifier 26 restoring
the electrical equilibrium of the electrode/tissue system
associated with the probe electrode 22 sufficiently to allow for
sensing of tissue depolarization within a few milliseconds
thereafter. In embodiments where sense amplifier 27, coupled to the
ring and can electrodes is used for capture detection, the capture
detect window can begin approximately 10 ms after the ventricular
pacing pulse and may end up to 80 to 100 ms thereafter. In
embodiments employing amplifier 26 for capture detection, the
associated capture detect window would typically have to begin
somewhat sooner (e.g. 5-8 ms following delivery of the pacing
pulse) to reflect the fact that the induced depolarization waveform
as sensed between the tip and can electrodes occurs more closely
following the pacing pulse.
Use of amplifier 26 for capture detection purposes is feasible,
especially in those cases in which capture is achieved with lower
pulse amplitudes so that the amplifier is capable of restoring the
equilibrium condition at the electrodes within about 5 ms or less
following the pacing pulse. The operation of the pacemaker as
illustrated to deliver the lowest energy pacing pulse that reliably
effects capture assists in accomplishing this result.
The time interval from the termination of the ventricular pacing
pulse to the start of the electronic capture detect window is
referred to herein as T1. At the expiration of T1, the capture
detect window T2 begins. The T1 period begins at the conclusion of
the ventricular pacing pulse. The duration of the T1 period should
be short and experimentation suggests that in systems employing
field density clamp sense amplifiers, 5-10 ms is an appropriate
value. The duration of period T2 should be long enough to allow
detection of any pacemaker triggered depolarization.
Experimentation suggests that 30-100 ms is an appropriate duration
for T2.
A capture detect signal (ECD) is generated when the sense amplifier
26 generates an SD signal during the capture detect window T2. This
capture detect signal may be used in a variety of ways, and is
illustrated in the context of an auto-threshold type pacer. In this
instance, the capture detect signal ECD is communicated to
auto-threshold logic 35 via line 37. Auto-threshold logic 35
controls the energy content of the pacing pulses delivered by the
pulse generator 34 to the lead system. In the event that a pacing
pulse is delivered and no capture detect signal follows,
auto-threshold logic 35 will generate a control signal on line 45
to increase the amplitude or duration of the voltage signal defined
by pacing pulse waveform generator 34, correspondingly increasing
the amplitude or duration of the current provided to electrode 22.
Auto-threshold logic 35 may also decrement the amplitude or
duration of the defined voltage signal in response to an extended
period in which all pacing pulses successfully capture the heart to
enable a determination of the minimum energy required to reliably
pace the heart. Auto-threshold logic 35 may also respond to the
failure of a pacing pulse to capture the heart by quickly
triggering an additional pacing pulse at an increased energy level,
and may continue to trigger increasing energy level pulses until
capture is achieved, as illustrated in FIG. 6, below.
Examples of known apparatus for adjusting the energy content of the
pacing pulses generated by pulse generator 34 are disclosed in U.S.
Pat. No. 4,858,610 issued to Callaghan et al, U.S. Pat. No.
4,878,497 issued to DeCote, all of which are incorporated herein by
reference in their entireties. Of course, in the present invention,
adjustment will have to be made by varying the defined voltage
signals provided to the amplifier 26 rather than by the specific
circuitry disclosed in these patents, but the general methodologies
disclosed may still be employed.
Alternative pacing functions which may be modified in response to
the detection or non-detection of cardiac depolarizations during
the capture detect window are described in U.S. Pat. No. 4,795,366
issued to Callaghan et al., and in the above cited U.S. Pat. No.
4,305,396 issued to Wittkampf, both of which are incorporated
herein by reference in their entireties.
FIG. 5 is a block diagram of a microprocessor based pacemaker
employing the present invention. Operation of the field density
clamp amplifier 26 in conjunction with electrodes 22 and 24 and of
pacing pulse waveform generator 34 correspond to the operation of
the same components discussed above in conjunction with FIGS. 3 and
4. Amplifier 26 provides a sense detect signal on line 32 which is
provided to the pacer timing and logic circuitry 300. The analog
signal from the operational amplifier within FDC amp 26 is provided
on line 118 to an analog to digital converter 306, for storage and
waveform analysis.
Pacer timing and logic circuitry 300 includes programmable digital
counters and associated logic for controlling the intervals
associated with cardiac pacing functions. Most importantly, pacer
timing logic 300 includes the escape interval timer, the electronic
capture detect window timer and the autothreshold logic illustrated
in FIG. 4. The particular intervals timed by pacer timer logic 300
are controlled by microprocessor 308 via address/data bus 302. In
response to detection of a depolarization wavefront as indicated by
a logic signal on SD line 32, pacer timing logic 300 resets the
pacing escape interval timer therein, and initiates any other
timing functions that may be desired, under control of
microprocessor 308. These may include capture detect windows T1 and
T2. On expiration of the ventricular escape interval, a trigger
signal is generated on line 4 which triggers pacing pulse waveform
generator 34 to deliver a voltage waveform on line 47, as defined
by microprocessor 308 via address/data buss 302. For example, in
response to the failure of amplifier 26 to detect a depolarization
wavefront during electronic capture detect window T2, as
communicated to microprocessor 308 via address/data parts 302,
microprocessor 308 via address/data parts 302, microprocessor 308
may schedule the immediate delivery of a second pacing pulse at the
expiration of interval T2 or shortly thereafter, with an increased
amplitude or duration. In response to detection of a sensed
contraction during the electronic capture detection window T2,
microprocessor 308 may instead instruct pacer timing logic 300 to
begin the next subsequent ventricular pacing interval. Similarly,
microprocessor 308 may specify occasional gradual reductions in
output pulse energy level in order to determine whether the pacing
pulses being delivered have an adequate safety margin, as discussed
in some of the patents cited above. The analog output from
amplifier 26 is provided to an A-D converter 306, which operates
under control of microprocessor 308 via address/data bus 302. This
structure allows for storage of the output from amplifier 26 during
and following generation of stimulus pulses, and in response to
detection of spontaneous or induced cardiac depolarizations. For
example, a portion of random access memory 310 may be configured as
a recirculating buffer, with the digitized output of amplifier 26
stored therein under control of direct memory access circuitry 312.
For example, the previous 200 or 300 milliseconds of digitized
signal may be present in the recirculating buffer at all times. In
response to either delivery of a stimulation pulse or sensing of a
depolarization wavefront, microprocessor 308 may freeze the
recirculating buffer memory 100 or 200 milliseconds thereafter, and
transfer the contents of the buffer to a separate location within
memory 310 for later analysis. In this fashion, the waveform of the
signal on line 118 corresponding delivered stimulus pulse, the
waveform corresponding to the operation of the amplifier 26 to
restore the electrode-tissue system to its previous equilibrium
state and the induced or spontaneous depolarization waveforms may
all be retained for analysis using curve fitting or other forms of
digital waveform analysis.
While the embodiment illustrated in FIG. 5 takes the form of a
pacemaker, the signal storage and analysis circuitry discussed may
equally well be employed in the context of an implantable
antitachycardia pacemaker, an implantable cardioverter or an
implantable defibrillator, as discussed above, with results of
waveform analysis used to distinguish normally conducted and
ectopic beats, etc.
The operation of the invention in the context of an auto-threshold
pacemaker as discussed above is illustrated in FIG. 6, in tracings
1-6.
Tracing 1 corresponds to the voltage signal at the output of the
amplifier 26 on line 118 (FIG. 3), illustrating the pulses
delivered to the electrode 22 and R-waves indicative of cardiac
depolarization.
Tracing 2 corresponds to the voltage signal at the output of an
operational amplifier corresponding to amplifier 38 in FIG. 2,
having its inverting input coupled to ring electrode 21 and its
non-inverting input coupled to the pacer can 24. Tracing 3
illustrates the logic level output of the sense amplifier 26 (FIG.
5) on line 32 and the pulses illustrated therefore correspond to
the SD signals discussed above in conjunction with FIG. 4. Tracing
4 corresponds to the logic level output of amplifier 27 on line 39
(FIG. 4) and similarly indicates the occurrence of sensed
ventricular depolarizations.
Tracing 5 corresponds to the signals on line 43 from capture detect
timer 33 (FIG. 5). High logic level signals in tracing 4 thus
correspond to the durations of the capture detect windows T2 and
the spacings between the delivered pacing pulses (62,63,72,73,74)
and the T2 windows correspond to the T1 intervals.
Tracing 6 corresponds to the output of the ventricular pacing pulse
waveform generator 34 (FIG. 5). The energy level of the pacing
pulses delivered is reflected by the height of the pulse markers.
The occurrence of pacing pulses is also reflected by the artifacts
62,63,72,73 and 74, which extend across tracings 1-5 and correspond
to the output of amplifier 26 (FIG. 4) on during delivery of the
pacing pulses.
The first cardiac waveform 60a, 60b results from a normal sinus
depolarization of the heart. SD signal 61 on tracing 2 and the SD
signal 65 on tracing 4 reflect the detection of this event. In the
context of the pacer of FIG. 5, this detected depolarization resets
the escape interval timer 30. At the conclusion of the escape
interval, timer 30 generates a V-pace signal which triggers a
ventricular pacing pulse.
Artifact 62 and pacing pulse marker 69 on tracing 5 indicate the
delivery of a pacing pulse. A capture detect window T2 is defined
thereafter as indicated at 67, on tracing 5. No depolarization
results, as the pacing pulse is of insufficient energy to capture
the heart. This lack of capture is evidenced by the fact that no
V-sense detect signal on tracing 4 follows the delivery of the
pacing pulse at 62. In this instance the auto-threshold logic 35
(FIG. 4) generates another ventricular pacing pulse as indicated by
artifact 63. The amplitude of this pacing pulse is increased, as
indicated by pacing pulse marker 70 in tracing 6.
In this instance the second pacing pulse captures the heart as
evidenced by the depolarization waveform 64a, 64b on tracings 1 and
2. This ventricular depolarization was detected within the capture
detect window 68 following the delivery of the pacing pulse at 63,
as evidenced by V-sense detect signal 66 in tracing 4. The tracings
associated with depolarization waveform 71a, 71b illustrate a
sequence of three pacing pulses delivered at 72,73,74. The first
two pacing pulses fail to capture the heart, as indicated by the
absence of V-sense detect signals thereafter in tracing 4. Pacing
pulse energy is increased with each pulse, as indicated by pacing
pulse markers 80,81,82. The third pulse delivered at 74 is
successful in capturing the heart as indicated by V-sense detect
signal 76.
The embodiment of the invention illustrated in FIG. 6 is assumed to
employ the sense amplifier 27 to perform capture detection. For
this reason, there are no SD signals illustrated on tracing 3
following pacing beats, as the R-wave detector 110 (FIG. 4) is
assumed to be inhibited during and shortly following delivery of
the pacing pulse. However, if amplifier 26 were to be employed to
perform capture detection, SD signals corresponding to those at 66
and 76 would also be illustrated on tracing 3, following pacing
pulses 63 and 74, and would correspond to the output of AND gate 41
(FIG. 4).
While the embodiments discussed above all employ field density
clamp amplifiers to both sense depolarizations and to deliver
stimulus pulses, the field density clamp amplifier may also be used
to deliver stimulation pulses in conjunction with prior art type
sense amplifiers coupled to the stimulation electrodes. In these
circumstances, some blanking of the sense amplifier may be
required, however, the ability to deliver voltage pulses of
arbitrary waveforms is retained, as well as the ability to return
the electrode-tissue system to a state in which depolarizations can
rapidly be sensed following stimulus pulses.
In addition, while the embodiments disclosed are ventricular
pacemakers, the invention may equally well be practiced in the
context of an atrial pacemaker or a dual chamber pacemaker which
paces and senses in both the atrium and the ventricle. Similarly,
while the pacemakers disclosed in the present application are
pacemakers intended to treat bradycardia, the present invention may
also be practiced in conjunction with an antitachycardia pacemaker,
an implantable cardioverter or an implantable defibrillator.
Similarly, the present invention may also be valuable in
conjunction with nerve stimulators or muscle stimulators in which
delivery of stimulation pulses triggered by sensing of nerve or
muscle impulses is desirable or in which arbitrary output waveforms
or waveforms which are not readily accomplished by means of a
traditional capacitor-type output circuit are desired. Therefore,
the scope of the claims that follow should not be construed to be
limited to the specific embodiments disclosed herein.
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